1. Field of the Invention
This invention relates to an intake system for a multiple-cylinder engine, and more particularly to an intake system for a multiple-cylinder engine which is arranged to be supercharged by a kinetic effect of intake air.
2. Description of the Prior Art
There have been put into practice two types of multiple-cylinder engines which are arranged to be supercharged by kinetic effect of intake air, one being a type supercharged by an inertia effect of intake air and the other being a type supercharged by a resonance effect of intake air.
From the viewpoint of compactly arranging the engine, the latter type is advantageous over the former type in that the former type must be provided with a surge tank which is inherently large in size while in the latter type, the surge tank is not necessarily needed. The engine of the latter type will be referred to as a "resonance-supercharged engine" hereinbelow.
In the resonance-supercharged engine, the engine speed range in which the resonance effect of intake air becomes maximum depends upon the length of the resonance tube which is formed by a part of the intake passage. That is, as the length of the resonance tube increases, the engine speed range in which the resonance effect of intake air becomes maximum is lowered (See Japanese Patent Publication No. 60(1985)-14169, for example.). Accordingly, in order to obtain a satisfactory resonance effect in a low engine speed range, the effective length of the intake passage must be large. On the other hand, in order to obtain a satisfactory resonance effect in a high engine speed range, the effective length of the intake passage must be small.
Thus, it has been difficult to obtain a resonance-supercharged engine in which a satisfactory supercharging effect can be obtained in both a low engine speed range and a high engine speed range.
Further, the length of the resonance tube differs from cylinder to cylinder, that is, the distance between the intake port and the pressure reflecting portion differs from cylinder to cylinder by the distance between adjacent intake ports. If the effective length of the intake passage is shortened in order to enhance the supercharging effect in a higher engine speed range, the difference in the length of the resonance tube becomes larger, and imbalance of the pressure waves acting on the cylinders is enhanced, thereby making it difficult to improve the volumetric efficiency of all the cylinders.